Certain 2-(2'-substituted)benzoyl-1,3-cyclohexanedione herbicides are described in U.S. Pat. No. 4,946,981 and U.S. Pat. No. 5,006,158.
One preferred group of the above-described herbicidal compounds have the following formula ##STR1## wherein R.sup.1 is nitro, chloro or bromo; R.sup.2 is hydrogen or C.sub.1 -C.sub.4 alkyl; R.sup.3 is hydrogen or C.sub.1 -C.sub.4 alkyl; R.sup.4 is hydrogen, C.sub.1 -C.sub.4 alkyl or ##STR2## wherein R.sup.a is C.sub.1 -C.sub.4 alkyl; or R.sup.3 and R.sup.4 together form an alkylene having 3-6 carbon atoms; and R.sup.5, R.sup.6, R.sup.7, and R.sup.8 are each independently hydrogen or C.sub.1 -C.sub.4 alkyl.
These herbicides can be prepared by reacting a mole of dione having the structural formula ##STR3## wherein R.sup.3 through R.sup.8 are as defined above with a mole of 2-(substituted)-4-(alkylsulfonyl)benzoyl chloride having the structural formula ##STR4## wherein R.sup.1 is nitro, chloro or bromo and R.sup.2 is hydrogen or C.sub.1 -C.sub.4 alkyl.
A 2-(substituted)-4-(alkylsulfonyl)benzoyl chloride may be generated from its corresponding benzoic acid. It is possible to generate a benzoic acid from an alkyl-substituted benzene.
The oxidation of an alkyl-substituted benzene to its corresponding carboxylic acid (as shown in Scheme I) is a widely performed reaction and is known to occur by a variety of oxidants and under various reaction conditions. ##STR5## For instance, catalytic liquid-phase oxidations of toluenes are generally performed with molecular oxygen and catalytic amounts of cobalt acetate, manganese acetate and bromide ions in acetic acid as a solvent. This method is exemplified in U.S. Pat. No. 5,175,351, wherein alkanesulfonylalkylbenzenes are oxidized to alkanesulfonylbenzoic acids using molecular oxygen in acetic acid and/or propionic acid. The reaction is carried out in the presence of a catalyst containing cobalt and bromine ions and optionally also manganese ions and also magnesium and calcium ions. One disadvantage of that method is the requisite high pressure. For instance, Example 1 of that patent describes air being introduced at 16 bar pressure. This rigorous condition of high pressure is necessary to effectuate oxidation using this method.
In addition, EP 594,027 discloses the oxidation of ring-substituted alkyl benzenes to the corresponding carboxylic acids by the use of nitric acid as an oxidant. This reaction using nitric acid as an oxidant requires high temperatures (between 130.degree. and 170.degree. C.) and the use of a vanadium or cobalt catalyst.
J. Org. Chem. 1986, 51, 2880-2883, discloses the oxidation of ring-deactivated methylbenzenes to the corresponding carboxylic acids in a two phase (organic liquid/aqueous) system. The oxidation of the organic substrate is effected by aqueous sodium hypochlorite as primary oxidant in the presence of catalytic amounts of ruthenium tetraoxide (RuO.sub.4). The RuO.sub.4 is formed in situ by the action of sodium hypochlorite on RuCl.sub.3. This procedure also requires the presence of a phase-transfer catalyst, in this case, a quaternary ammonium salt. Two phase systems are difficult to work with because vigorous stirring is required to obtain efficient and consistent reaction rates. Further, the introduction of a phase transfer catalyst is a limitation because of the possibility of forming emulsions upon workup. Use of two phases also decreases reaction volume; hence, there is a concomitant reduction in productivity. In addition, RuCl.sub.3 is expensive and even if efficient recycle methods are developed, there are still considerable costs from lost ruthenium. Additional problems may also arise with ruthenium in the waste stream.
Japanese Patent Publication 57/200353 describes the oxidation of sulfonitrotoluenes with hypochlorites in the presence of a heavy metal peroxide and alkali hydroxides to generate sulfonitrobenzoic acids. The limiting aspects of this procedure are at least twofold. First, the presence of heavy metals (nickel) in the reaction mixture will require expensive disposal steps and second, the presence of peroxides may possibly lead to explosive mixtures.
It has now been surprisingly found that oxidation of 2-(substituted)-4-toluenesulfonic acids to the corresponding 2-(substituted)-4-sulfobenzoic acid by use of sodium hypochlorite may be accomplished without a metal catalyst and a phase transfer catalyst. Unexpectedly, the reaction can be carried out in a single aqueous phase. The fact that no additional catalysts are needed greatly reduces the costs of production and disposal of spent catalysts.